Men With Older Brothers ‘More Likely To Be Gay’, Study Says

by : Francesca Donovan on : 12 Dec 2017 12:25

People have often wondered what governs the sexuality of the estimated 10 per cent of people who make up the gay community.


In a ground-breaking study, scientists have tentatively offered a possible explanation based in biology – or nature – rather than nurture.

The research was inspired by one of the great unexplained quirks of biology: the more older brothers a man has, the more likely he is to be gay, apparently.


Now scientists claim having a male child triggers an immune reaction in the mother that can alter the brain of the growing baby.


The authors of the report, led by Toronto University, published their study in Proceedings of the National Academy of Sciences, and were quick to stress this is one of many factors in what determines a person’s sexuality.

They wrote:

Gay men have, on average, a greater number of older brothers than do heterosexual men, a well-known finding within sexual science. This finding has been termed the fraternal birth order effect.

Strong scientific interest in sexual orientation exists because it is a fundamental human characteristic, and because its origins are often the focal point of considerable social controversy.


They claim:

Our study is a major advance in understanding the origins of sexual orientation in men by providing support for a theorized but previously unexamined biological mechanism – a maternal immune response to a protein important in male foetal brain development.

The research hoped to explain what they call ‘one of the most reliable correlates of male homosexuality: older brothers’, which has seen the 3 per cent chance of a male son being gay doubling to six per cent if he has brothers.

Max PIxel

Boys are made up of an X and Y chromosome, whereas girls have two X chromosomes.

When a mother is pregnant with her first baby boy, her body reacts against the Y chromosome, and creates a large quantity of the antibody called anti-NLGN4Y.

These antibodies can affect the brain development in later male children, which the study authors have inferred to ‘suggest an association’ with the brain structure, and ‘subsequent sexual orientation in male offspring’.


They add:

This effect becomes increasingly likely with each male gestation, altering brain structures underlying sexual orientation in their later-born sons…

After statistically controlling for number of pregnancies, mothers of gay sons, particularly those with older brothers, had significantly higher anti-NLGN4Y levels than did the control samples of women, including mothers of heterosexual sons.


However, they were careful to admit the study isn’t proof. Thus, the scientific endeavour to decipher what ‘makes’ a person heterosexual or homosexual will continue.

It is worth noting, they say, a fraternal birth order (FBO) effect exists in the sexual orientation of men, but not of women.

Presumably members of the gay community will get on with their lives, hopefully able to avoid being put under the microscope for their sexuality.

Francesca Donovan

A former emo kid who talks too much about 8Chan meme culture, the Kardashian Klan, and how her smartphone is probably killing her. Francesca is a Cardiff University Journalism Masters grad who has done words for BBC, ELLE, The Debrief, DAZED, an art magazine you've never heard of and a feminist zine which never went to print.

Topics: Life, LGBT


  1. PNAS

    Male homosexuality and maternal immune responsivity to the Y-linked protein NLGN4Y